Temperature compensated resonant cavity



Aug. 7, 1962 w. A. SCHANBACHER 3,048,803

TEMPERATURE COMPENSATED RESONANT CAVITY Filed March 16. 1959' 2Sheets-Sheet 2 n Tillllllf ut mama,

ilnited States Fatent 3,048,803 TEMPERATURE CGMPENSATED RESONANT CAVITYWilliam A. Schanbacher, Los Angeles, Calif., assignor to Hughes AircraftCompany, Culver City, Calif., a corporation of Delaware Filed Mar. 16,1959, Ser. No. 799,773 9 Claims. (Cl. 333-83) The present inventionrelates to microwave resonant cavity structures and, more particularly,to such cavity structures having elements providing automatictemperature compensation to prevent changes in the resonant frequency ofthe cavity within the structure.

Numerous temperature compensation schemes for microwave cavities havebeen devised to provide a constant operating frequency; however, ingeneral, such schemes do not provide the degree of accuracy over thewide range of temperature changes required by present day applications.This has principally led to the use of materials having a lowcoefiicient of the thermal expansion for the entire cavity, therebyrendering other temperature compensation means unnecessary. Suchmaterials are generally diflicult to machine and, additionally, areheavier and more expensive than standard waveguide materials, such asbrass or aluminum.

It is, therefore, an object of the present invention to provide aresonant cavity having a temperature compensation structure permittinguse of standard waveguide materials for the major portion of the cavity.

Another object is to provide a resonant cavity, princi pally of standardwaveguide materials, with automatic linear adjustment of the length ofthe cavity for variations in the diameter due to temperature changes.

Still another object of the invention is to provide a temperaturecompensating mechanism adaptable for automatically and linearlyadjusting the length of a resonant cavity in response to diameterchanges due to temperature variations.

In accordance with the invention a resonant cavity is provided withstructure automatically and linearly adjusting the length of the cavityas the diameter varies because of temperature changes, and the like, tomaintain the resonant frequency at a substantially constant value. Suchstructure includes at least two elements having an angular contactingsurface therebetween. One of the elements is of the same conventionalmaterial as the cavity wall material, such as brass or aluminum;however, the material of the other element is one having a differentcoeflicient of thermal expansion, such as lnvar. The sum of the anglesof the contacting surfaces, where more than one is utilized, isestablished to provide axial movement between the elements in responseto temperature changes and establish a substantially linear change inlength for a change in diameter of the cavity. The

value of the resonant frequency of the cavity then remains substantiallyconstant for variations in temperature.

Other objects and advantages will be apparent from the followingdescription considered together with the accompanying drawings in which:

FIG. 1 is a perspective view of one embodiment of a resonant cavityassembled in accordance with the present I invention;

of FIG. 2;

FIG. 4 is a perspective view of a second embodiment of the presentinvention;

"ice

FIG. 5 is a partially exploded perspective view, partly in section, ofthe embodiment of FIG. 4; and

FIG. 6 is a perspective longitudinal cross section of the plunger ofFIG. 5.

Referring to FIG. 1 in detail, there is shown a cylindrical resonantcavity structure 11 having a separate upper portion 12 and lower portion13 suitably held together by a plurality of tension springs 14, extendedbetween holes 16 in the two portions. For propagating microwave energyto and from the cavity structure 11, waveguides such as rectangularwaveguides 17 and 18 are respectively mounted on end plates 19 and 21 ofthe two portions 12 and 13.

When energy is to be propagated through waveguides 1'7 and 18 in adominant, plane polarized, mode and the cavity of structure 11 isexcited in a circularly polarized mode, the broadwalls of each of thewaveguides 17 and 18 are extended parallel to the end plates 19 and 21with electromagnetic coupling provided therebetween by couplingapertures 22 and 23, respectively. Such apertures 22 and 23 are disposedcentrally through the end plates 19 and 21 and through the respectiveadjacent broadwall of the waveguides 17 and 13 between the longitudinalcenter line and one side thereof. For convenience and rigidity of themechanical connection between the end plates 19 and 21 and thewaveguides 17 and 18, respectively, the latter are suitably secured ingrooves 24 of the former.

in accordance with the invention the upper portion 12 of the cavitystructure 11 comprises several different elements adapted for assemblyinto a unit. Thus, there is provided an inner cylinder 26 having anannular flange 27 at the uppermost portion. A first ring 31 having aninside diameter substantially the same as the outside diameter of theinner cylinder 26 is provided as a slip fit for alignment purposes. Theouter peripheral surface of the first ring 31 is provided with threads33 to engage similar threads on the inner peripheral surface of a secondring 34, which serves as a spacer between the first ring and a thirdring 36. So that the relative positions of the three rings 31, 34, and36 may be readily adjusted, matching threads 37 are also provided on theouter peripheral surface of the second ring 34 and on the inner surfaceof the third ring 36. The threads 37 have a different pitch than thethreads 33 for adjustment purposes, as will be set forth hereinafter.

The third ring 36 also has an internal shoulder 38 and a secondinternally threaded portion 39 of greater diameter for receiving athreaded retaining ring 41. The upper end plate 19 has a ridge 42 on theupper surface, against which the retaining ring 41 abuts as it isthreaded into threaded portion 39 of the third ring 36. A first recessedportion 43 is included in the under surface of the upper end plate 19and the outermost edge thereof rests against a thin annular diaphragm 44of conductive material supported by the shoulder 38. Such annulardiaphragm is flexible and has an inner diameter equal to the insidediameter of inner cylinder 26.

For establishing electrical properties of the cavity within thestructure 11 the diaphragm 44 is electrically connected to flange 27, asby suitable soldering procedures, or the diaphragm may be unitary withthe flange of inner cylinder 26. Thus, in assembly a narrow opening 46(see FIG. 3) remains between the diaphragm 44 and the adjacent wall ofrecessed portion 43 of the upper end plate 19, and this opening islimited in depth to a distance substantially equal to a half wavelengthat the resonant frequency of the cavity (see FIG. 3). Also, upper endplate 19 is provided with a second re"- cesse'd portion '47, having adiameter substantially equal to the inner diameter of inner cylinder 26,which extends in depth for a distance substantially equal to a quarterwavelength at the resonant frequency of the cavity from the center ofthe opening 46. This latter quarter-wave dimension of the secondrecessed portion 47 establishes a current null of the microwaveexcitation at the mouth of the opening 46 and the former half-wavedimension of the depth of the opening establishes a low value ofimpedance at the mouth to minimize electrical losses at the junction ofthe upper end plate 19 and the diaphragm 44.

Now to assemble the upper portion 12 of the cavity structure 11, thefollowing procedure may be readily used. The three rings 31, 34, and 36may be threaded together in such order. The inner cylinder 26 is theninserted into the ring assembly until the diaphragm 44 is suitablydisposed to contact the internal shoulder 38 of the third ring 36.Finally, the upper end plate 19 is fitted to engage the diaphragm 44 andthe retaining ring 4-1 is threaded into the third ring 36 to therebyprovide a unitary upper portion 12 for the cavity structure 11. Thus,axial movement between the upper end plate 19 and the inner cylinder 26is provided by distortion of the diaphragm 44. Also, the length of thisportion 12 of the cavity is adjustable by the differential threadarrangement 33 and 37 of the three rings 31, 34, and 36 as by turningthe second ring 24.

The lower portion 13 for the cavity structure 11 may be readilyfabricated as a unit, as illustrated in FIG. 2, to provide lower endplate 21 with a substantially thinwalled cup portion 51. The lowermostinternal portion 52 of the cup 51 has a smooth surface and an insidediameter substantially the same as that of the inner cylinder 26. Thecup 51 is also provided with internal threads 53 to engage externalthreads 54 on the outer surface of the inner cylinder 26, such that acontinuous internal surface is provided between the lowermost portion 52of the cup and the inner surface of the cylinder when the two arethreaded together. This junction occurs at a quarter wavelength distancefrom the bottom of the cavity where 'a current null exists. Theuppermost portion of the cup 51 has a conical section 56 similar to aconical section 57 of the-lowermost portion of the first ring 3 1 andthese two sections are in opposing alignment. Conical sections 56 and 57will be referenced as cones here inafter. Prior to assembly of thecavity structure 11, a ring 61 is inserted between the two cones 56 and57. Such ring 61, which will be referenced as ring hereinafter, isillustrated in FIG. 2 to have two tapered portions 58'and 59, whichrespectively engage the two cones 56 and 57.

With the upper and lower portions 12 and 13 threaded together in themanner set forth in the foregoing, by means of threads 53 and 54 withthe ring 61 disposed between the cones 56 and 57 and with the tensionsprings *14 in place, there is provided a complete resonant cavitystructure 11. When the entire structure is made of one material, such asbrass or aluminum, with the exception of the ring 61, which is made of amaterial such as Invar having a substantially lower value of coefficientof expansion, linear temperature compensation is possible to maintainthe 'resonant frequency of the cavity substantially constant. Suchlinear compensation for temperature changes is based upon the fact thatthere is, in general, a relationship whereby the resonant frequency of acavity is a function of the cavity length and the cavity diameter andthat for higher order dimensional changes, such as those caused bythermal expansion, a very good linear approximation between the changein length and the change in diameter is possible to maintain thefrequency of the cavity constant. In accordance with the presentinvention, the approximation is embodied 1 in the slope of the tapers 58and 59 of the ring 61 and the matching cones 56 and 57, and this will beset forth in greater detail hereinafter.

Where it is desired to operate the cavity structure 11 with a vacuumcondition in the cavity, a tube 62 may be inserted through the upper endplate 19 to communicate with the cavity through the opening 46 andadapted for connection to a vacuum pump (not shown). Such operatingcondition also requires that conventional dielectric plugs 63 and 64 berespectively mounted within the two coupling apertures 22 and 23, sothat the cavity of structure 11 is sealed with respect to the input andoutput microwave system.

Now, with the cavity structure 11 suitably connected into a microwavesystem, an increase in the temperature causes the diametrical dimensionsof the cavity structure 11 to expand. The Invar ring 61 having a lowercoefjcient of thermal expansion remains substantially the same. Theresult is that the upper and lower portions 12 and 13 move toward eachother by virtue of the angular sliding engagement between the ring 61and cones 56 and 57 and the tension of the springs 14, so that thelength of the cavity decreases by suitable selection of the slopes ofthe angular sliding elements. The decrease in length distorts thediphragm 44 without altering the electrical properties of the cavity andcan be made substantially linear with respect to the increase indiameter, as previously stated. It is readily apparent that the converseresult occurs should the temperature of the cavity structure decrease.

A somewhat different structural arrangement utilizing the sameprinciples is illustrated in FIG. 4 of the drawings and reference ismade thereto. In the arrangement of such figure, a cylindrical resonantcavity structure 81 is provided with one coupling aperture 82 (see FIG.5) disposed centrally in the closed lower end of the cavity structurefor electromagnetically coupling energy between the cavity and arectangular waveguide 83 extended along a continuation of thelongitudinal axis of the cavity. A second coupling aperture 84 extendsthrough the peripheral wall of the cavity structure 81 toelectromagnetically couple energy between the cavity and a secondrectangular waveguide 86 (see FIG. 4) disposed with the broad wallsthereof parallel to the longitudinal axis of the cavity.

The uppermost portion of the cavity structure 81 is open-ended and hasinternal threads 87 extended between the upper end thereof and anintermediately disposed shoulder 83. To support a plunger 91 within thecavity structure 81, a circular plate 92 having peripheral threads 93engaging the threads 87 of the cavity structure is provided with acentral tubular extension 94 having internal threads 96. Such extension94 has a slightly converging taper and a plurality of longitudinal slots97, so that a threaded nut 98 applies holding pressure to an objectwithin the extension when threaded on external threads 99 of theextension.

In accordance with this form of the invention the plunger 91 comprisesseveral elements combined as a unit to provide suitable temperaturecompensation for maintaining the resonant frequency of the cavitysubstantially constant. Thus, as illustrated in FIG. 6, a circularcup-shaped element 101 having an outer diameter substantially the sameas the inner diameter of the cavity structure 81 is provided with acentral post 102 extended in the same direction as rim 103 of the cup.The dimension of such rim 103 is equal to a quarter wavelength toprovide a low impedance point (substantially a short circuit) betweenthe cavity wall and the lowermost portion of the plunger 91. A hollowtube 104 coaxially receives the post 102 and is provided with a circularflange 106 at the lower end and such flange has a diameter substantiallyequal to the inside diameter of the cup-shaped element 101.

The upper end of the tube 104 has external threads 107 which match theinternal threads 96 of extension 94 of plate 92 and an enlarged internalbore 108 to receive a plug 109. By suitably securing one end of atension spring 111 to the plug 109 and the other end to the post 102,when the post is disposed within the tube 104, the two elements aresuitably held together. To facilitate assembly and secure the spring 111to the post 102, such post is provided with a threaded bore 112 whichreceives a threaded plug 113 to which the other end of the sprlng issuitably secured. Two annular plates 116 and 117 having peripheralbevels 118 and 119, respectively, are disposed under circular flange 1%within the cup-shaped element 131 with the bevels facing each other inradial divergence. Between the two plates 116 and 117, there is disposeda third plate 121 having an enlarged peripheral rim 122 with angulartapers 123 and 124 on the respective innermost surfaces to slidablyengage the respective bevels 118 and 119 of the two plates. Hereinafter,the plates 116 and 117 Will be referenced as cones 116 and 117 and therim 122 as ring 122.

With the foregoing elements of the plunger 91 assem bled to have thestated relationships, the tension of spring 111 maintains the cones 116and 117 in sliding contact with the ring 122. When the cones 116 and 117are of conventional waveguide material, such as brass or aluminum, andthe intermediate ring 122 is of a material having a lower coeflicient ofthermal expansion, such as Invar, the relative vertical or axialposition of the three plates 116, 117, and 121 varies with changes 1ntempera; ture. By suitable selection of the angles of the cones 116 and117 and the ring 122, the overall longitudinal dimension of the plunger91 varies linearly with respect to variations of the diameter of thecones, and this fea ture will be described more fully hereinafter.

The plunger 91 is supported in the cavity structure 81 by plate 92 andnut 98 to provide the desired cavity dimensions. A cover 126 havingthreads 127 engag ng threads 37 of the cavity structure 81 suitablyseals the upper open end of the structure. To provide evacuation of theinterior of the cavity structure 81 conventional microwave iris windows131 (only one being shown) are provided at coupling flanges 133 and 134,respectively, of the waveguides 83 and 86 and one of the couplingflanges 133 is provided with a port 136 communicating with the interiorof the waveguide 83 for coupling to a vacuum pump (not shown).

The operation of the cavity structure 81 is substantially the same asthat previously set forth with respect to the structure 11 described inconnection with the embodiment of FIG. 1. Thus, in this latterembodiment, a change in temperature results in a change in the diameterof the cones 116 and 117 of the plunger 91 and such change in dimensioncauses a corresponding change in the length of the plunger to maintainthe resonant frequency substantially constant. In the first instance thelongitudinal dimension of the cavity within the structure 11 is linearlychanged by an automatic adjustment of the respective aligned positionsof the two separate portions 12 and 13 of the structure. In the secondinstance, the longitudinal dimension of the cavity within structure 81is linearly changed by an automatic adjustment of the length of theplunger 91 within the cavity. In both instances it is to be realizedthat the automatic adjustments are carried out in response totemperature changes and the like, which might otherwise alter theresonant frequency of the cavity.

As has been stated previously, the resonant frequency, F of a microwavecavity as a function of the cavity length, L and the cavity diameter, Dand that, for higher order dimensional changes such as those caused bythermal expansion and the like, a very good linear approximation betweenthe change in length and the change in diameter can be found such thatthe cavity frequency remains constant. The realization of the requiredchange in cavity length is accomplished by transforming the differentialradial expansion of the combination of elements having differingcoefficients of thermal expansion into an axial motion via the slopingsurfaces.

It follows mathematically from the foregoing that when the change inlength, dL equals a constant, C, times the change in diameter, dD thenthe change in the frequency, F,, is equal to zero. For an example of themathematical development of the foregoing, reference is made to theplunger 91 of FIG. 6 for illustration of certain angles, Z, anddimension, D. As the two similar cones 116 and 117 expand, the ring 122maintains substantially its same diametrical dimension. Therefore, thetwo cones 1116 and 117 are forced apart axially along the angle formedby the matching surfaces of the cones and the ring 122 and this axialmovement is dependent upon the size of the angle. For a morecomprehensive expression, the total number of degrees of the angle, orangles where more than one compensating combination is provided, mustfill the following equation:

where:

C:=constant from AF =O, when dL =C-dD N=number of angles of matchingsurfaces C =coeflicient of thermal expansion D =diameter of the cavityD'=etfective minor diameter of ring 122 (see FIG. 5)

Z =angle of ith matching surface measure from a perpendicular to thelongitudinal axis of plunger 91 (sec FIGS) While the foregoing analysishas been set forth in detail with respect to the compensatingcombination of elements for the plunger 91 of the cavity structure 81,illustrated in FIGS. 4-6, the same analysis holds for the compensatingcombination of elements for the cavity structure 11 of FIGS. l-3. Thus,there has been described in detail a combination of similar elements asapplied to two structurally different resonant cavity structures 1 1 and8 1 to provide automatic compensation of the dimensions of the cavityWithin the structure and thereby maintain a substantially constant valueof resonant frequency for the cavity under varying temperatureconditions. It is to be noted that the choice of materials discussed forvarious elements has been illustrative only and that other combinationsof materials are within the scope of the invention. Further, it has beenindicated in the foregoing that more than one combination ofcompensating elements having sloping surfaces may be readily utilized solong as the equation set forth above is satisfied.

Additionally, the compensating structure has been described andillustrated with respect to cones and rings for both embodiments andthese elements have matching angular or tapered portions, which are notto be limiting in any manner because many other contact arrangementsbetween the elements is feasible to provide the desired linear relationbetween changes in diameter and changes in length. Thus, where the ringis suitably tapered the cones may have a sliding contact therewith suchas by a rounded or pointed ridge, and vice versa. It will therefore beapparent that the controlling factors are the provision of at least twoelements having at least one angular surface along which the otherslides to provide a linearly related axial movement between the twoelements.

While the salient features of the present invention have been describedin detail with respect to two embodiments, it will be readily apparentthat numerous changes may be made within the spirit and scope of theinvention and it is, therefore, not desired to limit the invention tothe exact details shown except insofar as they may be set forth in thefollowing claims.

I claim:

1. Temperature compensation structure for -a resonant cavity having acylinder and a pair of parallel end walls separated by a predeterminedlength, said structure comprising at least one ring element disposednormal to the axis of said cylinder and having a first coefficient ofthermal expansion, at least one annular element having a surface thereofin sliding contact with a surface on said ring element and having asecond coefiicient of thermal expansion dilfering from that of said ringelement, at least one of said surfaces having a conical shape with theangle tihereof being proportional to the ratio of diametr-ical thermalexpansion and axial movement between said ring and cone elements withsaid expansion and movement being linearly related.

2. Temperature compensation structure for a resonant cavity comprisingat least one ring disposed normal to the axis of said cavity and havinga first coefiicient of thermal expansion, and a pair of conesrespectively contacting said ring with sliding engagements and having asecond coeflic-ient of thermalexpansion, said first coefficient beingdifferent than said second coefiicient, said sliding engagements beingconical with the sum of the angles being proportional to the ratio ofdiametrical thermal expansion and axial movement between said ring andcones with said expansion and movement linearly related.

3. In a temperature compensated resonant cavity, the combinationcomprising a cylindrical resonant cavity structure having a cavity ofpredetermined diameter and length and means included in said structurefor controlling said length in response to thermal changes in saiddiameter, said means including at least one ring disposed normal to theaxis of said cavity and having a first coefficient of thermal expansionand at least one cone mounted in contact with said ring and having asecond coefiicient of thermal expansion, said first coefficient beingdifferent than said second coefiicient, said cone and ring having anangular contacting .surface with the angle of such surface beingproportional to a constant equal to the ratio of changes in diameter andchanges in length of said cavity.

4. In a temperature compensated resonant cavity, the combinationcomprising a hollow metallic cylinder and a plunger mounted in saidcylinder to provide a resonant cavity, said plunger including elementsof different coefficients of expansion with conical contacting surfacestherebetween, the sum of the angles of such surfaces being proportionalto a constant equal to the ratio of changes in diameter and changes inlength of said cavity.

5. In a temperature compensated resonant cavity, the

combination comprising a hollow metallic cylinder and a plunger mountedin said cylinder to define a cavity, said plunger having two circularmembers movably mounted with respect to each other with a combination ofa ring element disposed between two cone elements mounted between saidcircular members, said ring and cone elements having differentcoefficients of thermal expansion and angular sliding contact surfacestherebetween, the sum of the sliding angles of said surfaces beingproportional to a constant equal to the ratio of changes in diameter andchanges in length of said cavity.

6. In a temperature compensated resonant cavity, the combinationcomprising a hollow metallic cylinder, a plunger having parallel upperand lower circular members mounted for axial movement of the lowermember with respect to the upper member, means for mounting said plungerwithin said cylinder with a fixed relation between said upper circularmember and said cylinder to define a cavity, and a ring element mountedbetween said upper and lower circular members With cone elementsrespectively disposed on either side of such ring element in contactwith said ring element and adjacent circular member, said ring elementhaving a different coefficient of thermal expansion than said coneelements and having angular sliding contact with said cone elements withthe sum of the angles of said sliding contacts being proportional to aconstant equal to the ratio of changes in diameter and changes in lengthof said cavity.

7. In a temperature compensated resonant cavity, the combinationcomprising a cylindrical resonant cavity structure having separate upperand lower portions and a first coefficient of thermal expansion, a ringhaving a second different coefiioient of thermal expansion disposedbetween said upper and lower portions with at least one angular slidingcontact surface therebetween, the angle of said surface beingproportional to a constant equal to the ratio of changes in diameter andchanges in length, and means disposed between said upper and lowerportions securing the portions together for axial adjustment by saidangular surface.

8. In a temperature compensated resonant cavity, the combinationcomprising a cylindrical resonant cavity structure having separate upperand lower portions and a first coefficient of thermal expansion, saidupper portion including adjustable annular elements flexibly supportingan inner cylinder of substantially the same inside diameter as that ofsaid lower portion, a ring having a second different coefficient ofthermal expansion disposed between one of said annular elements and saidlower portion with at least one angular sliding contact surfacetherebetween, the angle of said surface being proportional to a constantequal to the ratio of changes in diameter and changes in length, andmeans disposed between said upper and lower portions securing saidportions together for axial adjustment by said angular surface with saidinner cylinder secured within said lower portion.

9. In a temperature compensated resonant cavity, the combinationcomprising a cylindrical resonant cavity structure having separate upperand lower portions and a first coeflicient of thermal expansion, saidupper portion including adjustable annular elements, an inner cylinderhaving an inside diameter substantially the same as the inside diameterof said lower portion, a thin annular diaphragm mounted between saidannular elements and said inner cylinder for flexible support, at leastone ring disposed between one of said annular elements and said lowerportion with at least one angular sliding contact surface therebetween,the sum of the angles of said contact surfaces being proportional to theratio of diametrical thermal expansion and axial movement between saidportions with said expansion and movement being linearly related, andspring means disposed between said upper and lower portions togetherwith said inner cylinder secured within said lower portion.

References Cited in the file of this patent UNITED STATES PATENTS2,439,908 Rigrod Apr. 20, 1948 2,600,225 Ehrenfried June 10, 19522,824,258 Snow et al. Feb. 18, 1958 2,880,357 Snow et al. Mar. 31, 1959

